1. Field of the Invention
The present invention relates to a light source device, an optical scanning device, and an image forming apparatus. More specifically, the present invention relates to a light source device that emits laser light, an optical scanning device that includes the light source device, and an image forming apparatus that includes the optical scanning device.
2. Description of the Related Art
Image forming apparatuses employing a laser as a light source are widely used for electrophotographic image recording. Such image forming apparatuses include an optical scanning device that scans the surface of the photosensitive element with a light beam (a writing light beam) output from the light source and deflected by the deflector to form latent images on the surface of the photosensitive element. In image forming apparatuses, the light amount of a writing light beam is changed with changing temperature or over time, which may cause inconsistencies in density of final output images (output images).
A conventional optical scanning device employing an edge-emitting laser monitors light backwardly output from the edge-emitting laser and performs auto power control (APC) for suppressing light output fluctuation. However, a surface-emitting laser does not backwardly emit light because of its structure, and therefore, an optical scanning device employing a surface-emitting laser requires light amount control different from the conventional APC. A method for performing APC based on the output of a light detector that is obtained by branching a part of a light beam output from a surface-emitting laser using an optical element such as a beam splitter and a half mirror and guiding it to the light detector has been devised as a light amount control method using a surface-emitting laser.
For example, Japanese Patent Application Laid-open No. 2002-040350 discloses an optical scanning device in which while a deflector deflects a laser beam output from a surface-emitting laser and collimated by a collimator lens to subject a surface to be scanned to scanning exposure, a beam splitting unit reflects a part of the laser beam to make a photoreceptor detect a light amount. The optical scanning device includes an aperture that collimates the laser beam at a position between the beam splitting unit and the collimator lens.
Japanese Patent Application Laid-open No. 2007-079295 discloses a multi-beam light source apparatus that includes: a light source unit with a plurality of luminous sources monolithically arranged in a direction of main scanning; a coupling lens for converting a plurality of optical beams output from the light source unit into a predetermined convergence state; and a supporting member for supporting the light source unit and the coupling lens in one body. In the multi-beam light source apparatus, the supporting member includes: a first member that supports the coupling lens and is installed such that a rotational adjustment is possible for centering an optical axis of the optical beams output from the light source unit; and a second member that supports the light source unit and is installed so that an inclination of the first member in a main scanning plane can be adjusted.
Japanese Patent Application Laid-open No. 2009-065064 discloses a monitoring device that includes: an optical element having a first aperture arranged such that a first portion of the light beam output from a light source and having maximum intensity passes through a substantial center of the first aperture, and a reflecting portion arranged around the first aperture such that the reflecting portion reflects a second portion of the light beam as a monitoring light beam; an aperture member having a second aperture that shapes a beam diameter of the monitoring light beam reflected by the optical element; and a light-receiving element that receives the monitoring light beam coming through the second aperture.
However, the optical scanning device disclosed in Japanese Patent Application Laid-open No. 2002-040350 and the monitoring device disclosed in Japanese Patent Application Laid-open No. 2009-065064 have disadvantages such that the optical path length from the light source to the light detector becomes long to disturb downsizing of the devices.
Moreover, the multi-beam luminous source apparatus disclosed in Japanese Patent Application Laid-open No. 2007-079295 has disadvantages such that the accuracy of APC is largely influenced by the fluctuation of the divergence angle.
Typically, a semiconductor laser is mounted on a package, and a cover glass is provided at the front of the package for dust proofing. Light that is output from the semiconductor laser and is reflected by the cover glass enters the semiconductor laser as a feedback light and deteriorates the light stability of the laser. The deterioration of light-emitting properties due to the feedback light becomes prominent particularly in a semiconductor laser in which a plurality of light-emitting elements is closely spaced like a vertical cavity surface-emitting laser (VCSEL) array.
For this reason, removal of the cover glass can be considered. However, the removal causes disadvantages such that dust adheres to the semiconductor laser to lower the light output or to shorten the service life of the light source in a mounting and assembling process or under the conditions of use in the market.